A substrate with a transparent electrode, in which a transparent electrode layer composed of a conductive oxide thin film is formed on a transparent substrate such as a transparent film or glass, is widely used as a transparent electrode of a display, a touch panel, or the like. Principal factors that determine performance of a substrate with a transparent electrode include an electric resistance and a light transmittance of a transparent electrode layer, and an indium-tin composite oxide (ITO) is widely used as a material having both a low resistance and a high transmittance.
In recent years, a substrate with a transparent electrode, which includes a transparent electroconductive layer having a lower resistance than those in the past, has been needed as the size of screens of displays and touch panels have increased. Patent Document 1 describes that when the tin oxide content of ITO on a glass substrate is increased, the carrier density increases, so that an ITO transparent electrode layer has a reduced resistance. More specifically, in Patent Document 1, film formation is performed at a substrate temperature in a range of 230 to 250° C. using a target with a tin oxide content of about 10% by mass.
On the other hand, when a film is used as a transparent substrate, the substrate temperature cannot be raised during film formation in view of the heat resistance of the substrate. Therefore, when a film substrate is used, a method is widely used in which an amorphous ITO film is formed on a film substrate by a sputtering method at a low temperature (e.g. 150° C. or lower), and then heated/annealed under an oxygen atmosphere to transform the amorphous ITO film into a crystalline ITO film. However, there is the problem that as the tin oxide content of the ITO film becomes greater, an amount of time required for crystallization increases, and therefore productivity of a substrate with a transparent electrode is reduced, or crystallization is insufficient, so that a reduction of the resistance is hindered as described in Patent Document 2.
When an ITO film is formed on a glass substrate as a measure against the above-mentioned problem, the time required for crystallization can be reduced by annealing the ITO film at a high temperature of 200° C. or higher. However, since a film substrate cannot withstand such a high temperature, an ITO film formed on a transparent film substrate should be crystallized at a relatively low temperature of about 150° C., and thus it is not easy to improve productivity by reducing the time required for crystallization.
Patent Document 3 describes a method in which ITO having a high tin oxide content and ITO having a low tin oxide content are laminated to reduce the time required for crystallization. In the method in Patent Document 3, however, a sufficient reduction of the resistance of the ITO film after crystallization is hindered because ITO having a low tin oxide content is partially used. For laminating a plurality of ITO films having different tin oxide contents, a plurality of targets having different tin oxide contents should be used, which may cause a reduction in productivity and an increase in cost of production equipment.
Patent Document 4 describes that when the water partial pressure in a chamber before the start of formation of an ITO film and during formation of the ITO film is extremely reduced to 1.0×10−4 Pa or less, the time required for crystallization of the ITO film can be reduced. To achieve such a low partial pressure, moisture and gases adsorbed to a substrate film should be removed by reducing the chamber pressure before the start of formation of an ITO film. When the inside of the chamber is evacuated using a vacuum pump, the time required for evacuation exponentially increases as the ultimate pressure becomes lower (the ultimate vacuum degree becomes higher). For reducing the water partial pressure in a chamber to 1.0×10−4 Pa or less before the start of formation of an ITO film, a long period of time is needed for evacuation before film formation, so that the time required to complete film formation after introduction of a film substrate into the chamber (occupancy time of film deposition apparatus) increases, and therefore productivity as a whole tends to be reduced, although the crystallization time is reduced.